Packages That Protect Seeds

LOUIS N. BASS, TE MAY CHING, AND FLOYD L. WINTER.

PACKAGES for seeds used to be mere containers. Modern packaging uses dozens of methods and materials to keep seeds at their original quality from the time they are processed to the time they are planted.

How seeds are packaged affects their physical characteristics of size; weight; color; moisture content; purity (freedom from weed seeds, inert matter, and other crop seeds); and freedom from disease organisms, insects, rodents, and mechanical damage. Physiological aspects like viability, vigor, and dormancy are affected but not the genetic qualities, except under unusual conditions.

The best way to maintain good viability and vigor of many kinds of seeds is to store them in a dry, cold (near or below freezing) place.

Many kinds will retain good viability and vigor for several years even at quite high temperatures if they are kept very dry. But when dry seeds in porous containers (burlap, cotton, paper) are removed from refrigerated and dehumidified storage, they absorb moisture rapidly from the atmosphere and may absorb so much moisture that viability is impaired in a few days or weeks. If very dry seeds are stored in moistureproof containers, however, absorption of moisture when they leave storage is not a problem.

Very dry seeds of 3 to 8 percent moisture that are kept in moisture-proof containers retain good viability and vigor in various conditions of temperature and humidity. Moistureproof packaging therefore seems to be the most economical way to maintain the quality of processed seeds during storage and marketing.

Packages designed to protect most physical qualities of seeds are made of materials that have sufficient tensile strength, bursting strength, and tearing resistance to withstand the normal handling procedures. Such materials, however, may not protect seeds against either insects and rodents or changes in moisture unless special protective qualities are built into them.

Packages for processed seeds are made of burlap, cotton cloth, paper, films, metal, glass, fiberboard, and various combinations of materials. Some offer moisture protection. Others do not. Each material has characteristics that make it suitable for a particular type of package, however.

BURLAP is a low-cost fabric woven of good-quality jute yarn. Usually it is 40 inches wide and weighs 7 to 12 ounces a yard. Both cloth and bags are made in a variety of fabric constructions to conform to the buyers' specifications.

Burlap is strong in tensile strength and tear resistance. Burlap bags can be stacked high in storage and withstand rough handling in distribution. Burlap cloth usually holds its original qualities for several years under normal usage. Long exposure to water or dampness and to strong sunlight weakens burlap. The bags may be reused many times and still adequately protect most of the physical qualities of seed lots.

Burlap cloth is used in laminations with various other flexible materials, such as asphalt, films, and paper. Laminated burlap bags can be made that are resistant to moisture transmission, insects, and rodents. Some types of laminated bags retain the gases for fumigation and seed treatment. Many burlap and burlap-laminated bags are used for strength instead of their barrier properties.

COTTON BAGS for seeds are made from sheeting, printcloth, drill, osnaburg, and a special seamless material.

Cotton fabrics are produced in different widths and weights, each designed for a special purpose. Osnaburg, which is stout and coarse, and seamless fabrics have the greatest tensile strength and tear resistance of the cotton materials and are used most. Seamless cotton bags often are reused many times, but the other cotton bags normally are used only once.

Cotton fabrics can be coated and laminated. Selected laminating materials are bonded together by asphalt or vegetable or compounded latex adhesives; the cotton cloth provides the needed strength and. protective properties.

Cotton bags protect physical quality, but have no effect upon moisture content, insects, or rodents, unless special barrier properties are built in. The two most widely used laminated moisture proof bags consist of one or two sheets of paper attached to a fabric with asphalt or a compounded latex adhesive. Other moistureproof constructions utilize such barrier materials as vegetable parchment, Pliofilm, polyethylene, and rubber coatings. Moistureproof liners sometimes are used inside cotton bags.

PAPER PRODUCTS are used extensively for packaging seed.

Most small packets are made of bleached sulfite or bleached kraft paper, which is coated with white clay to facilitate printing. A typical paper stock for small packets has a basic weight of 70 pounds per 500 sheets (25 x 38 inches). The packets are designed to contain a measured amount of seed without loss, but not to protect viability under unfavorable conditions.

Many paper seed bags have multiple plies or layers. These multiwall bags can be constructed of various thicknesses of smooth or crinkled paper. The kraft paper commonly used in them has a basic weight of 40 to 50 pounds per 500 sheets (24 x 36 inches).

Multiwall bags of smooth paper are produced in a variety of constructions,each designed for a specific purpose. Regular multiwall bags consist of two or more plies of kraft paper. The outside ply is heavy to take wear. Special plies are hidden among the layers of the multiwall. When moisture protection is required, a special barrier material, such as asphalt, polyethylene, or aluminum foil, is included. Barrier materials can be used for any layer, but usually they are between the two outer layers of paper.

A new trend for materials that readily absorb moisture, such as seeds, is to place a thin film laminate on the inside surface of the inner ply. Sometimes a film laminate is applied to the outside surface of the outer ply.

Ordinary multiwall bags have poor bursting strength. When they are piled high, the bottom bags burst. The top bags in high piles often slip. Ordinary multiwall bags tend to dry out in dry climates and become brittle along folds and on the corners at wear points.

ELASTIC MULTIWALL paper bags have several walls of crinkled paper. The number of plies depends on the weight of the product to be packaged. Often the two outer plies are laminated together with asphalt to provide a proper moisture-vapor barrier. The moisture-proof outer layer also protects the inner layers of paper from damage by rain.

Elastic materials cannot be evaluated by the usual physical test data for tensile and tear strength because the entire principle of the elastic multiwall bag depends upon the stretchability of the paper. The weight of paper generally used (untreated per ply) is 45 pounds per 500 sheets (24 x 36 inches) in the flat or uncrinkled form. A 15-percent stretch gives a finished weight of about 52 pounds per 500 sheets.

The asphalt-laminated outer duplex ply consists of two sheets of 45-pound kraft held together with 40 pounds of asphalt laminate. Adding a 15-percent stretch to the outer ply makes a finished weight of about 150 pounds per 500 sheets.

This outer ply provides the moisture barrier to prevent wetting or moisture absorption of the seed within the bag. At 80 F. and relative humidity of 75 percent, this ply has a moisture-vapor transmission rate of 0.17 gram per 100 square inches per 24 hours. This layer resists passage of water indefinitely.

The duplex-ply paper bag is considered to be airtight, but there may be some interchange of gases through the top closure sewing holes.

The thickness, toughness, and stretch of the outer ply provides the elastic multiwall paper bag with excellent puncture resistance. The resilience of the elastic multiwall material absorbs the shock of impacts and keeps the bag from splitting.

SOME SEED BAGS are made of laminates of paper/polyethylene/aluminum foil. These combinations afford better protection against moisture than foil or polyethylene used alone with paper. Paper can be treated to repel insects and rodents.

Multiwall paper and laminated cotton and burlap bags are designed to meet specific requirements of a variety of conditions of weather, shipping, handling, and storage. Manufacturers therefore seldom supply the same type of bag for a particular seed throughout the country or even to two seedsmen in the same locality.

Seed companies do not use the same type of bag for all kinds of seed. They package seed, such as corn, in two or three types of bags for example, osnaburg, elastic multiwall paper with an asphalt barrier ply, and 10-mil (0.01 inch) polyethylene.

FILMS of cellophane, Pliofilm, polyester, polyvinyl, aluminum foil, and polyethylene are used alone or in various combinations.

Cellophane, made of regenerated cellulose, is produced in more than 100 varieties. Each is designed for specific purposes.

Moistureproof types, which have low moisture-vapor transmission rates, are used for small packages. Cellophane alone may become brittle with age or in dry localities and break easily, but several firms produce combinations of cellophane and polyethylene that do not become brittle, like cellophane alone, and offer quite good moisture protection.

Polyethylene - cellophane laminates heat-seal easily and perform well on automatic packaging machines.

Pliofilm is a thermoplastic rubber hydrochloride plastic film. It resists ripping, tearing, and splitting. It seals well at low temperatures, has good moisture-barrier properties, and can be laminated to itself, paper, foils, and other films. Pliofilm can be used on most packaging machines designed for flexible-film packaging. It may deteriorate in strong light.

Polyester films are heat-sealable, transparent, flexible plastic materials with low moisture-vapor, carbon dioxide, and oxygen transmission rates. They have great tensile strength. They will not dry out or become brittle with age because they contain no plasticizer. Polyester film can be laminated to itself and practically any other material. Its flexible laminates can be used with most flexible packaging equipment. A new construction that utilizes a base of light cotton fabric and metalized polyester film offers easier fabrication, stronger seals, and resistance to flex damage, rough handling, and pinholes.